1. Field of the Invention
The present invention relates to a vehicle braking system for electric, hybrid electric, and conventional vehicles having an actively controlled brake caliper retractor mechanism.
2. Description of Related Art
As vehicle manufacturers increase the fuel economy of conventional vehicles using internal combustion engines, so-called parasitic fuel economy penalties previously considered trivial must be reduced. One such parasitic loss is attributable to disc brake caliper drag which occurs as a result of intentionally maintaining the brake pads in light contact with the brake rotor at all times of vehicle operation to provide a familiar and consistent brake feel to the vehicle driver. Although small, this friction imposes a fuel economy penalty, especially at high vehicle speeds and contributes to brake wear.
For electric vehicles (EV) and hybrid electric vehicles (HEV), parallel and series regenerative braking systems have been investigated. In the parallel system, a fixed relationship is imposed between the regenerative braking power applied by the vehicle drive motor, vehicle speed, accelerator pedal position, and brake pedal pressure, with the driver maintaining control of the hydraulic brakes. In order to collect a large fraction of the braking energy available in typical urban driving, the regenerative braking force must be equivalent to that which would be applied by the driver using conventional disk brakes of the vehicle, but with no driver input from the brake pedal. This results in either unfamiliar vehicle characteristics (e.g. rapid deceleration when the accelerator pedal is released) or curtailment of regenerative braking in order to achieve a more familiar vehicle "coast down" behavior. A series regenerative braking system typically includes an additional brake-by-wire system to intervene in driver control of the hydraulic brakes and optimally partition the driver-demanded braking force between the vehicle electric drive motor and friction brakes. Although the series regenerative braking system is efficient and maintains familiar vehicle behavior, it is expensive and adds to vehicle weight.